Axial-flow compressors



April 10, 1956 A. A. LOMBARD 2,741,423

AXIAL-FLOW COMPRESSORS Filed March 7, 1952 4 Sheets-Sheet l April 1o,195e A. A. LOMBARD 2,741,423

AXIM-mow coMPREssoRs Filed March '7, 1952 4 Sheets-Sheet 2 April 10 1955A. A. LOMBARD 2,741,423

AXIAL-FLOW COMPRESSORS Filed March 7,' 1952 4 Sheets-Sheet 3 INVE/l/TCBApril 10, 1956 A. A. LOMBARD 2,741,423

AXIM-FLOW COMPREssoRs Filed March '7, 1952 4 sheets-sheet 4 'speed ofthe compressor.

United States Patent() AXIAL-FLGW CMPRESSORS Adrian Albert Lombard,Allestree, Derby,England, assignor to RoHS-Royce Limited, Derby,England, a British company Application March 7, 1952, Serial No. 275,292

Claims priority, application Great Britain March 14, 1951 7 Claims.l(Cl. 239-114) This invention relates to multi-stage axial-flowcompressors and has for its object to provide means whereby theefficiency of the compressor may be maintained over a wide speed rangeof operation. It is usual to design such a compressor to run at aparticular speed which is normally a high proportion of the maximumoperating The characteristics of the blades of the compressor, includingtheir incidence, pro- 4le and pitch, are therefore chosen to give'theoptimum eiiciency fat the speed at which the compressor is designed tooperate. At speeds below the design speed, when the pressure rise perstage is less than the designed pressure rise, there will tend to be anaccelerating ow 4from entry to exit of the compressor due to the designoverall density ratio not being achieved. This acceleration appears'as areduction of axial velocity at the entry of the compressor and anincrease at the exit.

It has been found that the reduction of axial velocity at the entry tothe compressor results in the rotor blades #of the initial stages of thecompressor, and also in some cases the stator blades of the initialstages, operating at a much higher angle of incidence than in the designcondition. When the speed of the compressor is reduced suiciently thishigh angle of incidence results in the blades stalling, the eiciency ofthe compressor then 'oeing seriously reduced. it has been proposed thatair should be bled oi from a later stage of the compressor, whereby theaxial velocity of the air passage Vthrough the earlier stages of thecompressor is increased.

This invention is concerned with multi-stage axial-flow compressorshaving bleed valve means for controlling passages through which air isbled o from a later stage of the compressor, and has for an object toprovide a method of controlling the flow conditions in such a compresserso as to maintain a high eiciency of compression through a wide speedrange.

According to this invention in one aspect, a method of controlling theilow conditions in a multi-stage axial-dow compressor which has bleedvalve means comprises, over part at least of the range of rotationalspeeds of the compressor, automatically and progressively varying theopening afforded by the bleed valve means so that, for each speed in therange in which control is eected, the bleed valve means has acorresponding opening to give eiicient operation of the compressor. Byopening is meant the eiective area of the aperture afforded by the bleedvalve, through which the working iuid is allowed to escape.

According to this invention in another aspect, there is provided thecombination with a multi-stage axial-dow compressor having bleed valvemeans, of means operative over part at least of the range of rotationalspeeds of the compressor for automatically and progressively varying theopening of the bleed valve means in a manner corresponding to changes inthe compressor rotational speed, so that, for each speed, the bleedvalve means 'ice vhas a corresponding opening appropriate to giveeicient operation of the compressor at'that speed.

In another aspect, the invention provides in combination 'a multi-'stage'axial-flow compressor, bleed valve means arranged to Ycontrol 'the dowof uid bled from the compressor, and speed-sensitive 'means sensitive tothe rotational speed of the compressor and connected to controlprogressively the opening of the bleed valve means so that, for eachrotational speed, the bleed valve means has a'corresponding openingappropriate to give eicient operation of the compressor at that speed.

In yet another aspect, the invention provides in Vcombination, amulti-stage axial-flow compressor having a bleed passage from a laterstage, a bleed Valve in said bleed passage to control the ilowtherethrough, a speedsensitive device including a member whose positionis dependent on the rotational speed or" said compressor in a range ofspeeds, and a connection between said member and said bleed valvearranged so that, when said member moves on increase of rotational speedwithin the said range, said bleed valve moves towards its closedposition and vice versa, so that, vfor each rotational speed in the saidrange, the bleed valve has a corresponding opening appropriate to giveetiicient operation of the compressor at that Yrotational speed.

According to another aspect of this invention, a multistage aXial-owcompressor comprises bleed valve means, pressure-sensitive meansarranged to control the opening of said bleed valve means, ysaidpressure-sensitive means being loaded in one sense by a iluid pressureload which is a predetermined function of the rotationai speed of thecompressor, and being loaded in the opposite sense bya force which is afunction of the opening of said bleed valve means, saidpressure-sensitive means being connected to control said bleed valvemeans so that, for each speed, the bleed valve means has a correspondingopening appropriate to give efficient operation of the compressor atthat speed.

By rotational speed is meant either the actual rotational speed or thecorrected rotational speed. The corrected rotational speed is equal7 asis well known in the art, to the actual rotational speed `multiplied bya constant and divided by the square root of the absolute temperature atinlet to the compressor.

According to a feature of the invention the bleed valve means maycomprise a slide valve.

According to another feature of the invention, the bleed valve means maycomprise a seating formed round an aperture in the compressor stationarystructure, w' ic aperture opens into a chamber in communication with theWorking uid passage of the compressor, and a varve member which isarranged to slide relative to the seating to open and close the aperturein the compressor stationary structure.

According to yet another feature of the invention, the multi-stageaxial-How compressor having bleed valve means and means forautomatically and progressively varying the opening thereof inaccordance with engine rotational speed, may also compriseadjustable-pitch stator blades and means for automatically andprogressively varying the pitch of the stator blades in accordance withengine rotational speed.

Conveniently the pressure-sensitive means for varying the opening of thebleed valve means may comprise a hydraulic ram of which the piston isloaded by a pressure drop which is dependent on the rotational speed ofkthe compressor against a resilient load so that the piston occupies aposition in its cylinder which is dependent on the rotational speed ofthe compressor. Thus on increase of the compressor rotational speed thepiston will be displaced from one position in the ram cylinder injwhichthe change.

. tween appropriate positions.

One such constructionof the means for varying the opening of the bleedvalve means, comprises a hydraulic ram having a piston with sides ofdilerent effective area which is connected with the bleed valve means sothat movement of the piston in the ram cylinder from one end to theother effects progressive variation of the opening of the bleed valvemeans, a pressure fluid supply connected to each side of the piston froma common source, the supply to the side of the piston of largereffective area being through a low-restrictor, an outlet valve forcontrolling the flow of pressure uid from that side of the piston fedthrough the flow-restrictor, a pressure-responsive device for operatingthe outlet valve, spring means applying to the pressure-responsivedevice inthe sense of closure of the outlet valve'a load which isdependent on the position of the ram piston in the ram cylinder andwhich increases withincreasein the compressor'rotational speed, and asecond pressure uid supply arranged to apply to the pressure-responsivedevice a load which opposes that due to the spring means and which is afunction of the compressor rotational speed.

With this arrangement, on, say, an increase in the compressor rotationalspeed, the load on the pressure-responthe pressure acting on this'sideof the piston. The piston therefore moves along the ram cylinderincreasing the spring load on the pressure-responsive device and therebyclosing the outlet` valve and restricting the flow of iiuid from theside of the piston having the larger effective area. iinid from the sideof the piston having the larger area is so restricted by the outletvalve that the loads acting on the piston balance one another. Itrwillbe seen that since the iiuid pressure loaden thepressure-responsive,device Vis a function of the compressor rotationalspeed, the springload required to close ofi the outlet valve to anextent necessary to achieve balance of the loads on the ram pistonincreases with'increase of the compressor ro-V opening of the bleedvalve means, the pressure-responsive device may be subjected to anadditional spring load in the sense of closure of the outlet valvetodetermine the lower limit of compressorY rotational speeds at which theMovement of the piston continues until the ow of i means becomeoperative to effect adjustment of the openi ing of the bleed valvemeans.

The invention has an important application in a multistage axial-flowcompressor for supplying pressure air to Vthe combustion Vequipment of agas-turbine engine.

Many such engines have the compressor and the turbine interconnected torotate at the same speed or at speeds proportional to one another andarerprovided with a fuel system having a variable-delivery pump of theswash-plate type driven from the engine, whereof the pump rotor isarranged to act as a centrifugalpump supplying pressure fluid to a fuelcontrol device acting as a governor to limit the maximum fuel supply andthus as a top speed governor for the engine. Y

According to a feature ofV this invention, in such enf gines thepressure uid pressurised by the centrifugal action Yof the rotor of thesWash-plate variable-(delivery Y Y Vanneesr Y device as a function ofthe compressor rotational speed. With such an arrangement the sides ofthe ram piston may both be connected to the delivery of the pump, theside of larger area being connected to the fuel delivery of the pumpthrough the ow-restrictor.

Some constructional arrangements of this invention will now bedescribed, by Way of example, with reference to the accompanyingdrawings, of which:

Figure l shows a gas-turbine engine which has a compressor embodying theinvention, K v

Figure 2, is a sectional elevation of the fuel Vpump employed in theengine of Figure 1,

YFigure 3 is a diagram showing the control means and bleed valves of thecompressor of Figure 1, Y

bleed valves of another embodiment, and

Figure 5 is a diagram showing the bleed valve and its operating means ofanother embodiment. Y

The invention is applied in these constmctionsto a multi-stageaxial-flow compressor 10 forming part of a gas-turbine engine comprisingalso combustion equipment 11 receiving compressed air from thecompressor V10 and a turbine 12 receiving combustion products from thecombustion equipment and arranged to drive ther compressor.

Fuel is supplied to the combustion equipment to be burnt in thecompressed air by means of a Well-known fuel system comprising anengine-.driven swash-plate variable-delivery pump 13 which comprises arotor 14 having a number of cylinders 15 formed therein containingplungers 16 which, on rotation of the pump rotor, are Y constrained toreciprocate in the cylinders 15 and thereby chamber 18. Y Y Y K The pumprotor 14 is formed with a central bore 19 leading from the suction sideof the pump to a series ofV vradial drillings 20 which place thencentralbore in cornmunication with an annular chamber 21 in the housing of thepump rotor. The pumpY rotor is thus arranged to act as a centrifugalpump delivering fuel to the chamber 21 and the pressure within thechamber 21 is Va function of the engine rotational speed and thus oftheV compressor rotational speed.

The multi-stage axial-dow compressor is provided with one or more bleedvalves 3) at an intermediate compressor stage through which workingfluid may escape whenV the bleed valves are open. It will be appreciatedthat when the valves are open, the mass ow,'and therefore the axialvelocity, of the working iiuid will be increased in the stages prior tothe bleed valves, 'as compared with when the valves Vare closed. it willbeapparent that a1- though the plural is used for convenience, a singlevalve'Y ture 36. The extent of the chambers and apertures in the fperipheral direction is preferably a comparatively small portion of thecircumference. Y n n Y Co-operating with the inner surface of the outerWall of each chamber is a sliding plate valve 37, sliding of which inthe axial direction is arranged to vary the area of the respectiveaperture 36 between the limitsrof a fully open and a fully closedposition. An operating shaft 38 is supported in bearings in the radialWalls 39 which bound Ythe chambers at their axially-spaced ends, Vtheshaft 38 being connected to each plate valve 37 and having a porpumpmaybe employed to loadrthe pressureLresponsive tion 40 extending beyond oneend of the chambers;

Figure 4 is a diagram showing the control means and Y In order that thecompressor may operate efficiently over a wide range of engine speeds,it is desirable that the opening of the .bleed valvesshouldlbeadjustedso that, for each speed, their opening is suitable togive a high eciency of the compressor. It is .thus desirable to arrangethat, as the engine speed increases, the bleed Valves are progressivelyclosed and for this purpose the following control arrangment for thebleed valves is provided.

The operating shaft 38 of the bleed valves is connected for adjustment,through a suitable linkage 41, to a hydraulic ram 42 and the hydraulicram comprises a cylinder 43, an operating piston 44 working within thecylinder, and a control mechanism whereby the position of the piston 44within the cylinder 43 is determined ,in accordance with the enginerotational speed. Since the position of the piston within the cylinderdetermines the opening of the bleed valves, the opening will thus bedetermined in accordance with the engine rotational speed.

The ram cylinder 43 is provided internally with a pair of bushes 45 inwhich the stem 46 of the piston 44 slides so that the piston stem isguided in the bushes, andthe piston head 47 is located intermediate theends of the stem 46 and between the bushes 45, so that the .bushes actas limit stops for the piston. The stem 46 is hollow and incommunication with the cylinder on the right hand side of the pistonhead 47. ln this way the eiective area of the right hand side of thepiston is substantially greater than that of the left hand side.

The cylinder is provided with a pressure fluid supply connection 50,which in this arrangement is connected to the delivery of theengine-driven swash-plate type variable-delivery pump i3, and bores 51,52 are taken from this connection to each side of the ram piston, Vthebore 52 to the side of larger effective area comprising a restriction 53the purpose of which will be clear from the following description.

Mounted on the end of the ram cylinder adjacent the side of the pistonhaving the larger eective area, there is provided the control mechanismfor the piston. This control mechanism comprises a chamber dividedintotwo compartments S6, 57 by a exible diaphragm 58, the compartment 57which is nearer the ram cylinder being connected by a duct 59 to thesuction side of the fuel pump i3 and the other compartment 56 beingconnected by a duct 6i? to the chamber 21 of the fuel pump .into whichpressure duid is delivered by the centrifugal action of the pump rotor.'the diaphragm 53 is thus loaded yin one direction (towards the ramcylinder) by a pressure which is a function of the actual rotationalspeed of the engine.

The diaphragm 5S is also arranged to be vloaded by a main compressionspring 62 accommodated within the hollow stern of the ram piston and forthis purpose the end wall of the ram cylinder is formed with anaxiallydirected neck 63 having a bore therein' containing a sliding pushrod 64, one end of which bears on the diaphragm 58 and the other end ofwhich carries an abutment 65 for the spring. The other abutment for thespring is formed integrally with the piston stem 46. The push rod 64slides in the bore in the neck 63 and suitable duid-tight seals areprovided to prevent leakage of pressure uid from the ram cylinder intothe chamber 57. It will -be clear that as the piston 44 moves to theright, the spring load on the diaphragm will increase and will bedependent upon the position of the piston head 47 within the ramcylinder 43. It will also be clear that the spring load will oppose theuid pressure load on the diaphragm 58.

The diaphragm 5S is also arranged to be loaded by a secondary spring 66in the same sense as by the main spring 62. The secondary spring 66 hasone abutment on the diaphragm 58 and its second abutment on a shoulderformed within the neck 63 containing the push rod 64.

Movements of the diaphragm 58 under control of the tluid pressure andspring loads are communicated by a second push rod 67 mounted slidablyin a bush 68 fitted in the wall of the diaphragm chamber to a half-ballvalve member `69 which cooperates with a seating to Vformahali-.l'ialloutlet valve Vcontrolling the iiow of pressureuid ffromarpassage 76 communicating with the bore 52 which leads to theright-hand end of the ram cylinder. The second push rod 67 is shaped toprovide a fluid seal between the diaphragm chamber 56 and a chamberV 71accommodating the outlet valve. This latter chamber 71 is connected withthe suction side of the fuel delivery pump. The half-ball carrier andthe second push rod are lightly spring-loaded into engagement with thediaphragm. The operation of the hydraulic ram is as follows. YWhen theengine is stationary the half-ball valve 69 will be closed and thepiston head 47 will be at the left hand end of the ram cylinder 43,where it lis held by the main spring '62. yIf now the engine is startedup the pressure on each side of the piston head will increase, but sincethere is no leak from the right hand end of the cylinder through theoutlet valve 69, the pressures will be equal and the piston will remainstationary. At the same time the duid pressure load acting on thediaphragm 58 to move it to the left will increase due to increase inengine speed.

When the duid pressure acting on the diaphragm has increasedsufficiently to overcome the secondary spring 66 and thereby to permitthe half-bail outlet valve 69 to open, pressure duid is bled ed from theright hand end of the ram cylinder and the pressure within the cylinderon this side of the piston will fail due to the presence of therestriction 53 in its feed bore. When the pressure at this end of thecylinder falls, the piston will start to move to the right within thecylinder and will continue `to move so long as the engine rotationalspeed increases,

until is reaches the limit of its travel.

'It will be seen that the secondary spring 66 acts to .atord a lowerlimit to the range of speeds Vover which the ram 42 is eiective toadjust the opening of the bleed valves 30.

Assume now that the engine speed is steady within the range of speedover which ythe ram operates to adjust the opening of the bleed valvesand the engine speed is to be increased to a new speed in this range. Asthe speed increases, the pressure load acting on the diaphragm 53 willincrease permitting the half-ball valve 69 to open which allows vfluidto leak away from the right hand end ofthe Vrain cylinder. rl'hepressure on this side of the piston therefore falls (due to the presenceof the restriction 53 in its feed bore) and the piston moves within theram cylinder gradually increasing the spring load alforded by the mainspring 62 on the diaphragm until it is equal to the pressure load on thediaphragm. Increase of the main spring load tends to close the halfballoutlet valve 69 and to cut down the leak dow from the ram cylinder and,when the piston head 47 reaches a position in the cylinder appropriateto the new engine speed, the half-ball outlet valve 6% will have beenclosed off to such an extent that the loads acting on the piston arebalanced and the piston will stay in this position. This position is theone in which the spring load on the diaphragm 58 will balance the fluidrassure load derived from the centrifugal pump formed in the fuel pumprotor.

From the foregoing description it will be seen that for each engineactual rotational speed of the engine, and therefore of the compressor,the piston 44 occupies a corresponding position in the ram cylinder 43,so that the opening of the bleed valves 3) controlled by the hydraulicram can be arranged to be that appropriate to the particular enginespeed; moreover, for progressive changes of speed, the opening or" thebleed valves will be adjusted progressively.

If it is desired to control the opening of the bleed valves to be afunction or corrected engine speed (i. e. actual speed multiplied by aconstant and divided by the square root of the engine intaketemperature), an arrangement as shown in Figure 4 may he employed inwhich the diaphragm 58 is not loaded by the pressure fund-delivered byVthe,V centrifugal rmmp formed Vingthe fuel'pump rotor as abovedescribed, but is loaded by the Ypressure drop across avariable-area'oriice 100, of which A*the area is controlled by a capsule101 containedY in a ichamber 102 connected through the capillary tube103 to a bulb S in the engine intake.

'Ihe capsule 101 expands and contractsrin response to variations in theengine intake temperature. The orifice 100 is in a conduit 109 fed byanengine-driven feed capacity positive-displacement pnmp 104 driven atengine speed or at a speed Y proportional to engine speed, the whole ofVthe pump output passing through the oritice. The inlet of pump 104Y isconnected to duct 105',r which may be the supplyduct 'i connectionisopened by'the .actionfof the hydraulic ram 42, so the pressure tendingto close the valve 120 will l be progressively reduced, and the.effective area ofnthe to the engine fuel pump, and the conduit 109 maydischarge back into duct $.05 downstream of the oriice 100. A tapping106 from conduit 1%9 upstream of the orice 100 is taken to thecompartment 56 of the diaphragm beV proportional to the square root ofthe engine inlet temperature, it can be shown that the pressurediierence applied across the diaphragm 53 is proportional to the actualspeed divided by the square root of the inlet temperature, i.re. to thecorrected speed.

The arrangement of the present invention for controlling the adjustmentof bleed valves may be used in combina-V tion with an arrangement forautomatically and progressively varying the pitch of the stator bladesin accordance Vwith engine speed. For instance, as shown Yin Figure 1,

thesarne hydraulic ram 42 may be used to control both the bleed valvesand the stator blades, the piston of theY ram 42 being connected througha linkage 110 to a crank 111 which adjusts the angular setting oftherstator blades at the inlet of the compressor 10. v In anotherembodiment shown in Figure 5 vthe bleed VVvalve instead of being asliding plate valve maybe of the form of a piston valve, that is, apiston 120 sliding in a cylinder 121 and cooperating with ports 122V inthe` said bleed valve means, so that, for each speed, the bleedVcylinder walls, the position or" the piston in the cylinder determiningthe uncovered area of the ports and thus the effective opening of theVbleed valve. That part of the cylinder to which the ports open (whenthe valve is in the position inwhich the bleed valves'are open) will beViu communication with the working :duid duct 123 of the compressor, andthe cylinder may conveniently be formed as part of the stator casing E24of the compressor. The

Vpart of the cylinder on the'other side of the piston is closed, and isconnected through a connection 125 to Va source of high pressure, forexample to the delivery duct 126 of*v the compressor, The piston may beformed with a skirt 127 extending into this part of the cylinder so thatno escape of the compressed air takes place through the ports when thebleed valves are closed. There is' a restriction 3.23 in the connection12S to the compressorV deliveryduct, and there is also provided a ventV129 con- Y Y Vto a hydraulic ram such as the ram 42 that is describedabove, by means of a suitable linkage such as 41. The bleed valve pistonmay be lightly spring-loaded by a spring 131 in the sense of opening thevalve.

In operation, when the valve 130m the vent connection Y 129 is closed,the bleed valve 12% is maintained in the closed position since thepressure on the side of the piston tending to close the valve, e. g.compressor delivery pressure, exceeds the pressure on the Vother side ofthe piston,

Ywhich will be the pressure at an intermediate stage in the compressor.However, as the valve 130 in the vent ports 1220i the bleed valve Vwillbe progressively increasedr, ,ItV is arrangedthat the elective area ofthe Y ports ofthe bleed valve is increased Vas the. speed 'of thecompressor decreases, and Vice versa, over the desired range of speeds.l t Y VIclaim:y Y Y Y Y Y, Y Yl. A multi-stage axial-ilow compressorcomprising an inlet, an outlet, a stationary casing, a workingiluidpassagreV extending Ythrough saidstationary casingy between said. inletand said outlet, bleed valve means including means defining an aperturein said stationary casingl com- Y muncatingrwith said passage at a stageof said compressor intermediate` said inlet and said outlet andincluding a moveable valve member to co-operate with said aperture,

` a cylinder, a piston in said cylinder, a connection between saidpiston andsaid moveable valve member, and control means to control theposition of said piston in said cylinder comprising pressure-sensitivemeans, means including a pump driven at a speed proportional to thespeed of said compressor to produce a luid pressure load on saidpressure-sensitive means which is a predetermined function of therotational speed of Jh e compressor, and resilient means in abutmentwith said pressuresensitive means and with said piston to produce aresilient load in opposition to said fluid pressure load, saidVresilient load being a Yfunction of the position of the piston in thecylinder, and thus of the opening of the bleed Yvalve means, and meansoperable on movement of said Vpressure-sensitive means in said one senseto Vadjust a pressure kacting on said piston in the sense to cause`closing of said bleedf valve means and operable on movement Vof saidpressure-sensitive vmeans in said opposite sense to adjust said pressureinthe sense to cause opening of valve means has a corresponding opening.Y 2. A multi-stageaXial-ow compressor as claimed in claim l, whereinsaid` pressure-sensitivemeans comprises a flexible diaphragm.

3. A multi-stage axial-ow compressor comprising an inlet, an outlet, aworking fluid passage extending bctween said inlet and said outlet,bleed valve means communicating with said passage at a stage of saidcompressor intermediate said inlet and said outlet, a l'iydraulicV ramhaving a cylinder, a piston in said cylinder having sides of differenteective arca and connected with the bleed valve means, so that movementof the piston in the ram cylinder from one end to the other causesprogressive variation of the opening of the bleedvalve means, a pressureiluid'supply, a direct* connection from said jsupply to thesmaller-effective-area side of the piston,

a restricted connection from said supplyvto the largereffective-areaside of the piston, an outlet valve in communication with saidlarger-effective-area side'of the piston, a pressure-responsive deviceconnectedtoV said outlet'V valve to operate it, spring means in abutmentwith said pressure-responsive device, a part connected to move with saidpiston and also abutting said Vspring means, whereby there is applied tosaid pressure-responsive device Vin vthe senseV of Vclosure of theoutlet'valve a Vresilient load which is dependent on the position of theram piston in the ram cylinder and which increases as the bleed valvemeans move toward the closed positiomand means including apump driven ata speed proportional to the speed of said compressor to apply to thepressure- Yresponsive device a luidV pressure load which opposes thatdue to the spring load andV whichV is a function of the compressorVrotational speed and `increases asV the speed increases, so that, foreach speed, the bleed valve means has a corresponding opening.` Q Y 4. Amulti-stage axial-dow compressorasclaimed in claim 3, comprising alsoadditional springmeans in abutment with said pressure-responsive device,and an kabutment for said additional spring means in tixed relation tosaid cylinder, said additional spring means loading saidpressure-responsive device in the sense of closure of the outlet valve,whereby the fluid pressure load which is a function of the compressorrotational speed must overcome the load of said additional spring meansbefore the outlet valve can open, and thus whereby the lower limit ofcompressor rotational speeds is determined at which the means becomeoperative to effect adjustment of the opening of the bleed valve means.

5. A multi-stage axial-dow compressor as claimed in claim 3 wherein saidmeans to apply to the pressureresponsive device a fluid pressure loadwhich is a function of the compressor rotational speed comprises a pumprotor, means defining a central bore in said rotor, means definingradial drillings in said rotor in communication with said bore, wherebythe rotor acts as a centrifugal pump, drive means interconnecting saidrotor and said compressor, and means to place said pressure-responsivedevice in communication with said drillings.

6. A multi-stage axial-dow compressor as claimed in claim 3 wherein saidmeans to apply to the pressureresponsive device a tluid pressure loadwhich is a function of the compressor rotational speed comprises atixedcapacity positive-displacement pump, drive means interconnectingsaid pump and said compressor whereby the delivery of said pump is afunction of the compressor speed, a discharge conduit from said pump, avariablearea orifice in said discharge conduit, and temperaturesensitivemeans connected to control the area of said orifice and adapted to besensitive to the intake temperature of the compressor, and meansincluding a pair of tappings connected to said discharge conduit atpoints respectively upstream and downstream of said orice to apply thepressure drop across said oriiice to said pressure-responsive device.

7. A multi-stage axial-flow compressor comprising a stator casing, aport in said stator casing between the first and last stages of saidcompressor, valve means cooperating with said port, a hydraulic ramcomprising a piston member and a cylinder member, the piston memberdividing the cylinder member into a rst space and a second space, andone of said members being connected to the stator casing, a linkage bywhich the other of said members is connected to said valve means, andmeans to vary the relative positions of said piston member and saidcylinder member as a function of the rotational speed of the compressorcomprising a source of uid pressure, a rst conduit and a second conduitconnecting said source to said lirst and said second cylinder spacesrespectively, a restriction in said second conduit, a ixed-capacitypositive-displacement pump, drive means interconnecting said pump andsaid compressor, whereby said pump is driven at a speed proportional tothe compressor speed, a source of uid connected to the inlet of saidpump, a third conduit connected to the outlet of said pump, an orificein said third conduit through which passes the whole delivery of thepump, a chamber, a pressure-sensitive device dividing said chamber intoa rst part and a second part, a fourth conduit connecting said thirdconduit upstream of the orifice with said iirst part and a fifth conduitconnecting said third conduit downstream of the orilice with said secondpart, whereby said pressuresensitive device is loaded by a pressurediierence proportional to the speed of the compressor towards saidsecond part, resilient means between said pressure-sensitive device andsaid piston member to load it by an amount dependent on the relativepositions of the piston member and the cylinder member away from saidsecond part, an outlet from said second cylinder space, and a valvecontrolling said outlet and connected to said pressure-sensitive device.

References Cited in the iile of this patent UNITED STATES PATENTS1,111,498 Rotter Sept. 22, 1914 2,398,619 Clark et al. Aug. 16, 19462,418,801 Bauman Apr. 8, 1947 2,478,423 Ponomare et al. Aug. 9, 19492,527,732 Imbert Oct. 3l, 1950 2,570,847 Ovens Oct. 9, 1951 2,653,754McDonald Sept. 29, 1953 FOREIGN PATENTS 214,954 Great Britain Sept. 4,1924 579,976 Great Britain Aug. 22, 1946 595,351 Great Britain Dec. 3,1947 900,371 France Oct. 2, 1944

